Carboxylic acid-modified polysilsesquioxane aerogels for the selective and reversible complexation of heavy metals and organic molecules

Microporous and Mesoporous Materials 312 (2021) 110759

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Carboxylic acid-modified polysilsesquioxane aerogels for the selective and
reversible complexation of heavy metals and organic molecules
C.R. Ehgartner a, V. Werner a, S. Selz a, N. Hüsing a, A. Feinle a, b, *
a
b

Paris-Lodron-University of Salzburg, Department of Chemistry and Physics of Materials, Jakob-Haringer-Str. 2a, 5020 Salzburg, Austria
McMaster University, Department of Chemistry and Chemical Biology, 1280 Main Street West, Hamilton, ON L8S 4M1, Canada

A R T I C L E I N F O

A B S T R A C T

Keywords:
Adsorption
Carboxylic acid
Heavy metal ions
Methyltrimethoxysilane
Organofunctional
Porous polysilsesquioxanes

Organofunctional porous methyltrimethoxysilane (MTMS)-based aerogels are attractive for various adsorption
purposes due to the combination of their unique properties such as low densities and high specific surface areas
with tunable and accessible functional groups that can coordinate to, e.g., heavy metals and/or organic dye
molecules in polar and non-polar solutions. Furthermore, the MTMS backbone gives these aerogels mechanical

strength, the ability to be dried under ambient conditions and ensures their non-degradability in aqueous media
and recyclability. Herein, we report the preparation of carboxylic acid-modified polysilsesquioxane aerogels via a
simple and straightforward acid-base catalyzed sol-gel approach by using MTMS and the novel and stable 5(trimethoxysilyl)pentanoic acid. In this surfactant assisted co-condensation approach, all parameters (concen­
tration, pH, and temperature) have been carefully designed to yield porous (porosities between 82% and 53%
and specific surface areas between 345 m2.g− 1 and 36 m2.g− 1), light (bulk densities between 1.38 g.cm− 3 and
1.16 g.cm− 3), and hydrophobic aerogels with accessible and reactive functional carboxylic acid groups (-COOH)
(accessible surface loading up to 0.19 mmol.g− 1) for the adsorption of heavy metals ions (Zn2+ and Cu2+) and
cationic dyes (methylene blue and rhodamine B). The maximum adsorption capacities obtained from Langmuir
isotherms were 154 mg.g− 1, 106 mg.g− 1, 111 mg.g− 1, and 78 mg.g− 1 for RhB, MB, Zn2+, and Cu2+, respectively.
An increasing content of carboxylic acid groups influences the morphology, specific surface area and adsorption
behavior of the synthesized aerogels. Optimized functionalized aerogels can be dried ambiently and show high
and reversible adsorption abilities of 87% over several cycles towards cationic dyes in aqueous media. Moreover,
these carboxylic acid-modified aerogels demonstrate excellent adsorption selectivity by adsorbing only positively
charged molecules from mixed dye solutions, making them ideal candidates for diverse adsorption processes in
polar and non-polar solutions.

1. Introduction
Freshwater pollution with either heavy metals or organic dyes has
been a major concern to human health in the last decade. On the one
hand the inefficient treatment of wastewater from industry and sanitary
leads to pollutant accumulation in ground water and soil and on the
other hand precious resources (like gold and silver ions) are lost for
further industrial development [1–3]. Versatile techniques, like
adsorption [4], filtration [5], membrane separation [6], ion exchange
[7], and electrolysis [8] have been used in the past for waste water
treatment. From the mentioned approaches the adsorption technique is
considered the most promising and efficient technique regarding ease of
operation, cost-effectiveness, and regeneration [9]. Nevertheless,

adsorbent materials which focus on selective and repeatable adsorption

are rarely reported, even though such adsorbents have major advantages
in separation and regeneration of dye mixtures and for diverse sensor
applications [10–13]. Therefore, there is a high need to design suitable
adsorbent materials with tunable surface functionalities that show high
affinity to specific adsorbates [14–16].
Specifically organofunctional mesoporous aerogels with high spe­
cific surface areas, high porosities, and low densities are very promising
materials in the field of separation science and selective adsorption
processes [17]. The surface chemistry of these aerogels can be tailored
with a high number of different organic functional groups (e.g. amino,
sulfonate, mercapto, and hydroxyl groups) for specific applications
ˇ
[18–21]. Standeker
et al. synthesized silica aerogels with mercapto

* Corresponding author. Paris-Lodron-University of Salzburg, Department of Chemistry and Physics of Materials, Jakob-Haringer-Str. 2a, 5020, Salzburg, Austria.
E-mail address: (A. Feinle).
/>Received 12 August 2020; Received in revised form 9 November 2020; Accepted 10 November 2020
Available online 18 November 2020
1387-1811/© 2020 The Authors.
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C.R. Ehgartner et al.

Microporous and Mesoporous Materials 312 (2021) 110759

moieties in a co-condensation process for the absorbance of copper (II)
and mercury (II) ions from aqueous solution [22]. Ali et al. grafted
aminopropyl groups onto a silica aerogel surface for the adsorption of
chromium (III) ions [23] and the group of Nakanishi reported flexible
organofunctional mercapto-modified silica aerogels for the adsorption
of gold ions [24,25].
Another promising functional group for diverse and selective
adsorption and other applications is the carboxyl group since this group
can form hydrogen bonds with organic and inorganic species. Under
basic conditions, the carboxyl group is deprotonated and the resulting
negatively charged carboxylate entity can then act as a ligand for the
complexation of a variety of metal cations and other positively charged
molecules [26]. Functionalization of mesoporous silica with carboxyl
groups is mostly performed via multiple post-synthesis processing steps
which is often associated with diffusion problems, limited attachment
sites, loss of homogeneity, and pore blocking [27]. Anhydride or iso­
cyanate groups can, for example, be grafted onto silica in a first step,
followed by subsequent hydrolysis reactions [28,29]. Only few reports

of carboxyl-modified silica materials prepared by a co-condensation
route have been published since a limited number of carboxylate
group-containing organosilanes is commercially available [30–32]. The
group of Gaber and the group of Jones synthesized highly ordered
carboxyl-modified mesoporous SBA-15 by a co-condensation of tet­
raethyl orthosilicate (TEOS) and water-soluble carboxyethylsilanetriol
sodium salt (CES) [33,34]. These materials did not show a significant
affinity towards metal ion adsorption (Cu2+), indicating that the
carboxyl groups were not available for further chemical modifications.
Lin et al. reported the synthesis of a carboxylic acid-modified, disulfide
containing organosilane which was employed in a co-condensation
approach with TEOS to yield mesoporous MCM-41 [35]. Notwith­
standing, the synthesis of the precursors was complex and required
multiple reaction steps, extraction and purification of the final product
led to esterification of the carboxylic acid.
In our previous work, we reported a simple one-pot synthesis of
different stable carboxylic acid derivatized alkoxy silanes [36]. The
carboxylic acid ligands showed a high affinity for europium(III) ion
complexation reactions [37]. One of the silanes, namely 5-(triethox­
ysilyl)pentanoic acid was further employed as a precursor molecule in a
co-condensation approach with TEOS to create organofunctional silica
particles with high specific surface areas. These particles showed
excellent adsorption abilities towards the organic dye methylene blue
[38]. Recently, our group has shown the successful co-condensation of
methyltrimethoxysilane (MTMS), 5-(triethoxysilyl)pentanoic acid and
the biopolymer silk fibroin to yield mechanically strong and highly
porous hybrid aerogels for diverse water-oil separation and thermal
applications [39,40]. Generally, as illustrated in our previous work,
aerogels based on co-condensation of MTMS with other organofunc­
tional silanes showed promising abilities in terms of hydrophobicity,

mechanical stability and flexibility, which makes them ideal candidates
as adsorbents for organic pollutants, oil spills and regeneration, and
continuous flow processes in polar and non-polar solutions [41]. The
properties of standard brittle hydrophilic aerogels, with a high number
of –OH surface groups would deteriorate in aqueous solutions over time,
which limits their practical applicability especially in terms of regen­
eration of the adsorbents. The hydrophobicity of MTMS aerogels,
resulting from their stable methyl surface groups ensures their
non-degradability when exposed to water in comparison to standard
hydrophilic silica aerogels and even allows the aerogels to be dried
under ambient condition [42]. In this work, we describe a straightfor­
ward one-pot synthesis of carboxylic acid functionalized silica aerogels
from a co-condensation approach of MTMS and 5-(triethoxysilyl)pen­
tanoic acid followed by either supercritical fluid extraction with CO2 or
ambient pressure drying. All aerogels were investigated in terms of
morphology, structural properties, successful incorporation of the
functional (-COOH) group, and the influence of an increasing amount of
the functional group. The chemical accessibility of the carboxyl group

was studied in detail by equilibrium and kinetic adsorption experiments
with heavy metal ions (Zn2+ and Cu2+) and organic molecules (methy­
lene blue and rhodamine B). Special emphasis is also put on the selec­
tivity and reversibility of the adsorption process and on the ambient
pressure drying of the organofunctional silica gels, which makes these
materials interesting for diverse applications.
2. Experimental details
2.1. Materials
Methyltrimethoxysilane (98% purity, MTMS), hexadecyl­
trimethylammonium bromide (98% purity, CTAB), 4-pentenoic acid
(97%), platinum (IV) oxide, zinc sulfate heptahydrate, eriochrome®

Black T, ammonium chloride, murexide, methylene blue (hydrate),
thiazole yellow G, Titriplex (III), sodium acetate, sulfosalicylic acid
dehydrate, sodium carbonate, sodium hydrogen carbonate, and meth­
anol (99.8%, MeOH) were obtained from Sigma Aldrich. Glacial acetic
acid (AcOH), copper(II)sulfate (anhydrous), iron(III)nitrate non­
ahydrate and urea were acquired from Merck. Ammonium hydroxide
(28% in H2O) and 2-propanol were procured from VWR. Trimethox­
ysilane (95%) was purchased from Acros Organics. Rhodamine B was
provided by Alfa Aesar.
2.2. Synthesis of carboxyl-modified aerogels
The detailed synthesis of 5-(trimethoxysilyl)pentanoic acid via a
platinum catalyzed hydrosilylation of pentenoic acid with trimethox­
ysilane is described elsewhere [36]. Carboxy-modified poly­
organosilsesquioxane aerogels were prepared via a co-condensation
approach of methyltrimethoxysilane (MTMS) with 5-(trimethoxysilyl)
pentanoic acid (TMPA). The amount of hydrolysable silicon centers was
kept at a constant value of 34.9 mmol, and an increasing molar% of
MTMS (10%, 20%, and 30%) was substituted by TMPA. In the first step
of the two-step acid-base approach CTAB and urea were dissolved in 10
mM aqueous acetic acid. MTMS and TMPA were slowly added to the
mixture under stirring and ambient conditions. The starting composi­
tions can be found in Table 1. Stirring was continued for 30 min before
the sol was poured into tightly sealed PS containers (Ø 17.2 mm; height
57.6 mm). Similar to the approach described by Kanamori et al. the
containers were placed in a ventilated oven for 4 d at 60 ◦ C to induce
gelation and aging [42]. For the removal of residual surfactants the aged
alcogels were solvent exchanged in methanol (double the volume of the
monoliths) with at least 8 h in between three subsequent solvent ex­
changes cycles. For supercritical drying, the alcogels were again solvent
exchanged with 2-propanol in the same approach as methanol. Super­

critical drying was conducted in a custom-built autoclave with CO2 at
45 ◦ C and in a pressure range between 80 and 90 bar. For ambient
pressure drying, the synthesized wet gel was washed three times with
methanol and then solvent exchanged with n-heptane three times. Af­
terwards, the gels were dried at room temperature for 3 d.
The samples are labeled as follows: The first two letters (MT)
correspond to the MTMS precursor molecule and the attributed number
gives the molar% of the silane used. The second letter refers to the
second silane used for co-condensation (T for TMPA) and the following
numbers correspond to the molar% of the portion of MTMS that was
replaced by TMPA. For example, the sample MT80-T20 was prepared
with 80 mol% MTMS and 20 mol% TMPA and in which the molar% are
related to the constant amount of hydrolysable silicon centers (34.9
mmol).
2.3. Determination of the functional group bulk loading
The bulk loading of the solids with carboxyl groups were determined
by using equation (1):
2


C.R. Ehgartner et al.

Microporous and Mesoporous Materials 312 (2021) 110759

Table 1
Starting compositions[a] and selected structural properties of the carboxy group functionalized polysilsesquioxane aerogels.
Sample

Molar Ratio
MTMS:TMPA

MTMS
[g]

TMPAa
[g]

Lb
[%]

ρbc

ρsd

[g.cm¡3]

Pe
[%]

SBET f
[m2.g¡1]

MT100-T0
MT90-T10
MT80-T20
MT70-T30

10:0
9:1
8:2

7:3

4.80
4.32
3.84
3.36

–
0.80
1.60
2.30

4.7
9.9
23.1
20.9

0.21
0.25
0.46
0.55

1.36
1.38
1.32
1.16

84.4
81.8
65.1

52.5

548
345
145
36

a
b
c
d
e
f

[g.cm¡3]

Other components: CTAB 0.6 g, urea 0.5 g, and 10 mM acetic acid 7 g.
Linear shrinkage during drying.
Bulk density.
Skeletal density.
Porosity calculated by using the equation P = [1-ρb/ρs]*100.
Specific surface area determined using the BET model.

Bulk Loading

(
)
mmol
[100 − W200−
=

g
M

1000 ]

×R

The metal ion content in all experiments was determined via com­
plexometric titration with EDTA after separation of the metal ions so­
lution from the solid adsorbent by filtration over a
polytetrafluoroethylene syringe filter with a membrane size of 200 nm.
Zn2+-complexometric titration: 15 mL of the metal ion filtrate (the
pH was adjusted to ~10 with an ammonia buffer solution (5.4 g
ammonium chloride and 35 mL 25% ammonia solution)) was pipetted
into an Erlenmeyer flask and Eriochrome Black T was added as a met­
alchromic indicator. The solution was titrated with a 0.05 M EDTA so­
lution. The titration was repeated 3 times.
Cu2+-complexometric titration: 15 mL of the metal ion filtrate (with
pH 8 from the adsorption experiment) was pipetted into an Erlenmeyer
flask and murexide was added as a metalchromic indicator. The solution
was titrated with a 0.05 M EDTA solution. The titration was repeated 3
times.
The amount of heavy metal ions or dyes adsorbed on the solid
samples was calculated based on equation (2):

(1)

M (g.mol− 1) is the molar mass of the degradable carboxyl group, R is
the molar ratio of TMPA to MTMS and W200-1000 (g) is the weight loss
between 200 and 1000 ◦ C (determined by thermogravimetric analysis).

2.4. Adsorption experiments
Organic Dyes. Batch adsorption experiments were conducted at RT to
determine the adsorption of methylene blue (MB) and rhodamine B
(RhB) on the synthesized carboxylic acid-modified samples. In a typical
experiment, 10 mg of the solid was mixed with 45 mL of the RhB dye
solution (c = 100 mg.L− 1) and 5 mL EtOH. The pH was adjusted to ~8
with a diluted ammonia solution (1 M). The mixture was shaken and
then left to equilibrate over a period of 3 d. In a second step, adsorption
equilibrium experiments were performed where 10 mg of MT80-T20
was mixed with 45 mL of the aqueous dye solutions (MB or RhB) of
different concentrations (1 mg.L− 1, 5 mg.L− 1, 10 mg.L− 1, 25 mg.L− 1, 50
mg.L− 1, 75 mg.L− 1, and 100 mg.L− 1) and 5 mL EtOH. The pH was
adjusted to ~8 with a diluted ammonia solution (1 M). The mixture was
left to equilibrate over a period of 3 d. Furthermore, kinetic adsorption
experiments were conveyed, were 10 mg of the silsesquioxane aerogel
sample was mixed with 45 mL of the dye solutions (MB or RhB) with a
concentration of 50 mg.L− 1 for certain time intervals (1 h, 2 h, 4 h, 24 h,
2 d, and 4 d) and 5 mL EtOH after the adjustment of the pH to ~8 with a
diluted ammonia solution (1 M). The dye concentrations in all experi­
ments were determined via ultraviolet–visible spectroscopy after sepa­
ration of the dye solution from the solid by filtration over a
polytetrafluoroethylene syringe filter with a membrane size of 200 nm.
Heavy Metal Ions. Adsorption equilibrium experiments were con­
ducted at RT to determine the adsorption behavior of different heavy
metal ions (Zn2+, Cu2+) on the synthesized carboxylic acid-modified
samples. In a typical Zn2+ ion adsorption experiment, 10 mg of MT80T20 was added to 45 mL of an aqueous Zn2+ ion solution of different
concentrations (10 mg.L− 1, 25 mg.L− 1, 50 mg.L− 1, 100 mg.L− 1 and 200
mg.L− 1) and 5 mL EtOH. The pH was adjusted to ~8 with an ammonia
buffer solution (5.4 g ammonium chloride and 35 mL 25% ammonia
solution). The mixture was shaken and then left to equilibrate over a

period of 3 d.
In a typical Cu2+ ion adsorption equilibrium experiment, 10 mg of
MT80-T20 was added to 45 mL of an aqueous Cu2+ ion solution (5 mg.
L− 1, 10 mg.L− 1, 25 mg.L− 1, 50 mg.L− 1 and 100 mg.L− 1) and 5 mL EtOH.
The pH was adjusted to ~8 with a diluted ammonia solution (1 M). The
mixture was shaken and then left to equilibrate over a period of 3 d.
Furthermore, kinetic adsorption experiments were performed. 10 mg
of MT80-T20 was added to 45 mL of the metal ion solutions with a
concentration of 100 mg.L− 1 (Zn2+) or 20 mg.L− 1 (Cu2+) and 5 mL EtOH
and kept for certain time intervals (0.5 h, 1 h, 2 h, 4 h, 8 h, 24 h, 48 h, 3
d, and 4 d). The pH was adjusted to ~8 with an ammonia buffer solution
for Zn2+ or with a diluted ammonia solution (1 M) for Cu2+ experiments.

Qe =

(C0 − Ce )xV
m

(2)

where Qe is the equilibrium adsorption capacity (mg.g− 1), C0 and Ce are
the initial and the equilibrium concentrations of the heavy metal ion
solutions/dye solutions (mg.L-1), V is the volume of the heavy metal ion/
dye solution (L) and m is the mass of the polysilsesquioxane aerogel used
(g).
2.5. Selective adsorption experiments
0.1 g of MT80-T20 was added to 10 mL of a mixture of either thiazole
yellow G (ThG) and MB (1:1, c = 2 mg.L− 1) and 1 mL EtOH or to a
mixture of thiazole yellow (ThG) and RhB (1:1, c = 2 mg.L− 1) and 2 mL
EtOH. The pH was adjusted to ~8 with a diluted ammonia solution (1

M). The mixtures were shaken and then left to equilibrate over a period
of 3 d. The dye concentrations were determined via UV–vis spectroscopy
after separation from the solid adsorbent by filtration over a polytetra­
fluoroethylene syringe filter with a membrane size of 200 nm.
2.6. Reusability experiments
0.01 g of MT80-T20 was added to 45 mL of a MB solution (c = 10 mg.
L− 1) and 5 mL EtOH. The pH was adjusted to ~8 by adding a diluted
ammonia solution (1 M). The mixture was shaken and then left to
equilibrate over a period of 3 d. In a second step, the solid sample was
separated from the dye solution by filtration and placed in diluted HCl
(pH = 1–2) for 24 h. The samples were repeatedly washed afterwards
with water. This process (adsorption of MB and the consequent washing)
was repeated 5 times. The dye concentrations in all reusability experi­
ments were determined via UV–vis spectroscopy after separation from
the solid adsorbent by filtration over a polytetrafluoroethylene syringe
filter with a membrane size of 200 nm.
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2.7. Characterization

cluster structure mentioned in previous reports [42], whereas the car­
boxylic acid-modified MTMS samples (MT90-T10, MT80-T20, and
MT70-T30, Figs. S2b–d) were obtained as opaque monoliths. The
microstructure changed significantly from very small particle-networks
(particle size 5–15 nm) with nano-sized voids (4–40 nm) for the sample

MT100-T0 and MT90-T10 to the presence of larger structures (15 and
40 nm) and voids in the upper nanometer range (100–300 nm) for
MT80-T20 and MT70-T30.
Besides changes in the appearance and morphology of the monoliths,
the bulk density, linear shrinkage, and the porosities were influenced by
the introduction of carboxyl groups as well (Table 1). The bulk density,
for example, increased from 0.21 g cm− 3 for the reference sample
(MT100-T0) to 0.55 g cm− 3 after the incorporation of 30 mol% TMPA
(MT70-T30) and the shrinkage increased from 4.7% for MT100-T0 to
20.9% for MT70-T30 leading to a decrease in the porosity from 84.4%
(MT100-T0) to 52.5% (MT70-T30). The specific surface areas of the
monoliths and pore sizes were calculated from nitrogen adsorptiondesorption measurements. The obtained isotherms are shown in
Fig. 1a and the calculated BET specific surface areas are listed in Table 1.
The isotherms of all prepared monoliths can be classified as Type IV
according to IUPAC classification. Carboxylic acid-modified MTMS
aerogels showed very narrow hysteresis loops (Type H3) and capillary
condensation occurred above p/p0 > 0.5. The shape of the hysteresis
loops indicate that the samples have a mesoporous character with the
possibility of additional macropores in the network system that are not
completely filled with the pore condensate [46]. The nitrogen adsorp­
tion intake showed no saturation at the relative pressure close to unity.
This can be a direct result of pores in the macroporous region, where
capillary condensation still takes place at p/p0 ~1. The hysteresis loops
of the MTMS/TMPA samples got less pronounced with increasing
modification degree of the monoliths with carboxyl groups, indicating
broader pore size distributions. The average pore diameter (Tables S2
and SI) increased, and the BET specific surface areas (Table 1) decreased
with an increasing amount of TMPA. For a MTMS-TMPA aerogel with a
molar ratio of MTMS to TMPA of 9:1 (MT90-T10) a specific surface area
of 345 m2.g− 1 and an average pore size of 38 nm was calculated,

whereas the specific surface area was decreased to 36 m2.g− 1 and the
pore size increased to 119 nm after an increase of the molar ratio of
MTMS and TMPA to 7:3 (MT70-T30).

The morphology of the samples was analyzed with a scanning elec­
tron microscope (Zeiss ULTRA Plus) operating between 2 and 5 kV with
an in-lens detector and by a transmission electron microscopy (TEM)
JEOL JEM F200 with a cold field emission electron microscope oper­
ating at an accelerating voltage of 200 kV with a TVIPS F216 2k by 2k
CMOS camera. Nitrogen adsorption and desorption isotherms were
performed using a Micrometrics ASAP 2420 at − 196.15 ◦ C. The specific
surface area (SBET) was calculated with the Brunauer, Emmett and Teller
5-point method in the relative pressure range of 0.05–0.3. Prior to the
measurement, the samples were degassed in vacuum for 12 h at 300 ◦ C.
Thermogravimetric analyses (TGA) were carried out using a simulta­
neous thermal analyzer (Netzsch STA 449C Jupiter). The samples were
heated from 25 ◦ C to 1000 ◦ C with a heating rate of 10 ◦ C/min and an
oxygen flow rate of 30 mL/min. Structural characteristics of the aerogel
samples were investigated using a FTIR-ATR spectrometer (Bruker
Vertex 70) over a wavenumber range from 500 cm− 1–4500 cm− 1.
UV–vis spectra were conducted on a PerkinElmer Lambda 750 device.
The maximum adsorption wavelength was used for further calculations.
The mass to volume ratio of the cylindrically shaped monoliths was used
as a basis to calculate the bulk density (ρb) of samples. The skeletal
density (ρs) was determined via helium pycnometry (Quantachrome,
Micro-Ultrapyc 1200e T). Equation P = [1- ρb/ρs]/100 was used to
calculate the porosity (P) of the samples.
3. Results
3.1. Preparation of carboxylic acid-modified polysilsesquioxane aerogels
The preparation of carboxylic acid-modified polysilsesquioxane

aerogels suitable for adsorption purposes requires a careful tuning and
understanding of the synthesis conditions, such as the reaction rates of
different silanes in co-condensation processes, the choice of suitable
catalysts, and the addition of appropriate surfactants [43]. The combi­
nation of a hydrophobic silane (MTMS) with a hydrophilic silane
(TMPA) in a sol-gel process is not a trivial task. Additionally, steric and
charge effects by the large functional moieties of TMPA, competitive
cyclization reactions and different hydrolysis and condensation rates
have to be overcome [44]. In our study we therefore used a surfactant
aided two step sol-gel reaction in which the hydrolysis occurred in
diluted acetic acid as weakly acidic medium and the polycondensation
was initiated by the use of urea as a weak base-releasing agent (at
temperatures above 60 ◦ C) [44,45]. A ternary phase diagram illustrating
the relationship between the functional silane TMPA, the surfactant
CTAB, the catalyst for hydrolysis (HOAc) and the resulting gelation
behavior and the appearance of the monolith is given in Fig. S1 (SI). As
seen in the ternary phase diagram, the composition of the sol is crucial
for the prevention of phase separation processes and the later appear­
ance and structural properties of the monoliths. There is a critical
amount of CTAB (at least >0.5 g under given reaction conditions) that
hinders phase separation for all investigated modification with TMPA. In
this study, we introduced carboxyl groups into MTMS based monoliths
and studied the influence of the modification reaction on the
morphology and structural properties of the materials as well as on their
adsorption behavior towards metal ions and organic dyes. The compo­
sitions of the sol for the preparation of the carboxylic acid-modified
MTMS monoliths are given in Table 1 and were chosen to give mate­
rials with low densities, high porosities, and high specific surface areas
(see Tables S1 and SI).
MT100-T0 (pure MTMS monolith without TMPA) was prepared as a

reference sample under similar conditions as the carboxylic acidmodified samples to investigate the influence of TMPA on the proper­
ties of the material. Digital, SEM and TEM images are shown in Fig. S2
(SI). The pure MTMS reference sample (MT100-T0, Fig. S2a) was
transparent and showed the typical globular-aggregated mesoporous

3.2. Determination and accessibility of the carboxyl group
The successful incorporation of carboxyl groups in the silica network
of MTMS/TMPA polysilsesquioxane aerogels was determined via FTIR
spectroscopy (Fig. 1b). For all samples, the most intense vibration bands
were in the range between 1017 and 1105 cm− 1 and can be attributed to
asymmetric stretching vibrations of the Si–O–Si bonds. The less pro­
nounced bands at 768 cm− 1, 1408 cm− 1, and 1279 cm− 1 correspond to
the vibration of Si–C bonds. The antisymmetric and symmetric stretch­
ing of the methyl C–H bond of the MTMS/TMAP samples was identified
at 2972 cm− 1 and 2928 cm− 1, respectively. For the carboxylic acidmodified samples (MT90-T10, MT80-T20, MT70-T30) a new band
– O stretching vi­
appeared at 1712 cm− 1 which is ascribable to the C–
bration of the carboxyl group [47,48]. It can be clearly seen, that an
increasing TMPA content and a corresponding increasing number of
carboxyl groups attached to the MTMS framework significantly in­
– O vibration proving the successful
creases the band intensity of the C–
incorporation. Furthermore, a small shoulder at 1735 cm− 1 that can be
associated to ester compound formation was detected [36].
The thermal stability of the prepared samples was investigated by
thermogravimetric analysis (see Figs. S3 and SI). The first weight loss
occurred in the temperature region between 25 and 200 ◦ C. It was
almost negligible for the unmodified sample MT100-T0 but increased
from 2% for MT90-T10 and MT80-T20 up to 4% for MT70-T30. This
weight loss can be attributed to physisorbed water on the poly­

silsesquioxane surface [49]. A further increase in temperature up to 700
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Microporous and Mesoporous Materials 312 (2021) 110759

Fig. 1. Nitrogen adsorption-desorption isotherms (a) and IR-ATR spectra (b) of carboxylic acid-modified MTMS samples.

C led to oxidation reactions and decomposition of the organic groups
(methyl, carbonyl) and to condensation reactions of the residual silanol
and alkoxy groups. A comparison of the actual and the theoretical
weight loss (see Table 2) shows negligible differences for the
MTMS/TMPA samples and correlates very well to the molar ratios of the
precursor molecules. The number of incorporated carboxyl groups (bulk
loading) was quantitatively determined and increased with an
increasing molar percentage of TMPA from 0.21 mmol.g− 1 for
MT90-T10 to 0.98 mmol.g− 1 for MT70-T30.
The accessibility of the functional group was shown by a complex­
ation reaction of the carboxyl group with Zn2+ and adsorption experi­
ments towards MB. The spectra of the sample MT80-T20 before and
after complexation to Zn2+ are compared in Fig. 2. A partial shift from
the carbonyl vibration from 1712 cm− 1 to 1580 cm− 1 was observed and
indicates the conversion of the carboxyl group into a carboxylate moi­
ety. The conversion is not complete since the Zn2+ ions are not able to
reach every carboxylate group due to the hydrophobic property of the
MTMS framework and possible pore blocking. The accessible surface
loading of the carboxy-modified samples was calculated from adsorption
experiments with MB and is summarized in Table 2. It can be clearly

seen that MT80-T20 has the highest number of accessible surface groups
(0.32 mmol.g− 1) which are almost half of the available bulk surface
groups (0.56 mmol.g− 1). The sample MT90-T10 is too hydrophobic for
the dye to reach all available adsorption sites and for the sample MT70T30, the low surface loading can be attributed to its low specific surface
area. The accessibility and successful adsorption of the dyes and metals
is also directly related to the pH value of the aqueous solutions and the
used acid or base. For the adsorption of methylene blue at different pH
values (pH = 2, 4, 7, 8, 9, and 10) see Fig. S4 (SI). The pH value was
adjusted with different concentrations of either ammonia or HCl. At a
pH above 7 most of the –COOH groups are in their deprotonated form
and exhibit a higher adsorption capacity than at a pH below 7 [26]. At
pH = 7 only parts of the carboxylic acid groups are deprotonated, and
the adsorption capacity is lower than at a pH value above 7. The pH
value of 8 was chosen for further metal and dye adsorption experiments

to ensure mild reaction conditions and due to the fact that higher pH
values (pH > 8) lead to the slow dissolution of the polysilsesquioxane
backbone, which is not preferable for recyclability of the adsorbent and
other intended applications [50,51]. Moreover, the complexometric ti­
trations of Cu2+ solutions is very pH dependent and requires a pH value
of 8. Additionally, the adsorption behavior of different bases (ammonia,
sodium carbonate and sodium hydrogen carbonate) at pH 8 was
analyzed (see Fig. S5 (SI)). It can be clearly seen that ammonia and
sodium carbonate show similar adsorption behavior, whereas the use of
sodium hydrogen carbonate as a base at pH 8 showed a weaker
adsorption behavior. On this basis ammonia at a pH of 8 was chosen for
further adsorption experiments.

◦

3.3. Adsorption studies
The above analysis revealed that the functionalized aerogel sample
MT80-T20 has a relatively high number of accessible carboxyl surface
groups and high specific surface areas, which makes them promising
candidates for the adsorption of organic molecules (electrostatic in­
teractions) and complexation reactions with heavy metals. The adsorp­
tion performance of different samples with increasing –COOH content
towards the organic dyes rhodamine B (RhB)/methylene blue (MB) and
the metal ions Zn2+ and Cu2+ was tested (Fig. 3a). The hydrophobic
samples were first wetted with a small amount of ethanol (V = 5 mL) to
ensure that the sample does not float on the aqueous dye or metal salt
solution and can interact with the respective dissolved molecules and
ions. The highest equilibrium adsorption capacity was reached for the
sample MT80-T20 towards RhB with a value of 104 mg.g− 1 and the
lowest for the unmodified sample MT90-T10 towards Cu2+ with 10 mg.
g− 1.
Additional adsorption studies were conducted for the MT80-T20
sample and the adsorption kinetics towards RhB, methylene blue (MB),
Zn2+ and Cu2+ was analyzed in detail (see Fig. 3b). Initially, the heavy
metal/dye removal occurred fast but showed saturation over a course of
3 d. Overall, the adsorption of metals ions occurred with a higher rate
than the adsorption of dye molecules. Pseudo-first and pseudo-second
order adsorption models were applied to characterize the adsorption
kinetics according to the following nonlinearized and linearized equa­
tions (3)–(6) [52,53].
Pseudo-first order nonlinear adsorption model:
)
(
(3)
Qt = Qe − exp− K1 t

Table 2
Comparison of the actual and calculated weight loss and bulk loading of the
carboxylic acid-modified polysilsesquioxanes and accessible surface loading.
Sample

Weight
Loss
[%]

Calc. Weight
Loss
[%]

Bulk
Loadinga
[mmol.g¡1]

Surface
Loadingb
[mmol.g¡1]

MT90T10
MT80T20
MT70T30

78.7

79.5

0.21

0.04

71.7

71.4

0.56

0.32

67.2

64.8

0.98

0.19

Pseudo-first order linear adsorption model:
log(Qe − Qt ) = log(Qe − K1

(4)

Pseudo-second order nonlinear adsorption model:

a

Calculated from the thermogravimetric data and equation 1.

b
Accessible surface loading, determined from adsorption experiments with
MB at a pH value above 7.

Qt =

K2 Q2e t
1 + K2 Q2e t

Pseudo-second order linear adsorption model:
5

(5)


C.R. Ehgartner et al.

Microporous and Mesoporous Materials 312 (2021) 110759

Fig. 2. IR-ATR spectra of MT80-T20 before and after the complexation with Zn2+.
Fig. 3. (a) Comparison of the adsorption perfor­
mance of the MTMS/TMPA polysilsesquioxane aero­
gels modified with an increasing percentage of TMPA
towards Zn2+, Cu2+, MB, and RhB. (b) Adsorption
kinetic curve for the metal ions (Zn2+ and Cu2+) and
dyes (RhB and MB) sorption on the aerogel sample
MT80-T20. (c) Pseudo-first order linear kinetic model
fit and (d) pseudo-second order linear kinetic model
fit for the experimental data of the adsorbed capacity
of RhB by MT80-T20 with increasing adsorption

time.

t
1
t
=
+
Qt K2 Q2e Qe

variance and normality assumptions of standard least squares [54].
Therefore, the kinetic models were also fitted in their nonlinear forms.
The nonlinear curve fitting of the pseudo-first order and pseudo-second
order equations were solved through the lsqcurvefit user-defined func­
tion in Matlab until resnorm minimization was achieved and are shown
in Fig. S7 a-d (SI) for the adsorption of the metal ions/dyes onto
MT80-T20. The best-fit sorption kinetic model was analyzed with the
statistical error function ‘Chi-square Test’ according to following equa­
tion [54].
(
)
i=n
∑
Qe, exp − Qe,cal
χ2 =
(7)
Qe,cal
i=1

(6)

Qe (mg.g− 1) is the adsorption capacities at equilibrium and Qt (mg.
g ) is the adsorption capacities at time t (h). The pseudo-first and
second order rate constants are K1 (1.h− 1) and K2 (g.mg− 1.h− 1),
respectively. The linear fitting of the pseudo-first order and pseudosecond order equations were solved in Origin 6.0 and are shown in
Fig. 3 c/d (for RhB) and in Fig. S6 a-f (SI, for MB, Zn2+, and Cu2+) for the
adsorption of the metal ions/dyes onto MT80-T20. The constants of the
models were calculated from the slope and intercept of the straight lines
and the linear regression coefficient (R2) was applied as an indicative of
model fittingness. The calculated correlation coefficients R2 (Table 3)
clearly indicate that the pseudo-second order model fits well with the
experimental data of the metal ion (Cu2+, Zn2+) and dye (MB, RhB)
adsorption with high R2 values between 0.991 and 0.999. The calculated
adsorption capacity Qe (pseudo-second order model, Table 3) addi­
tionally showed a good agreement with the experimental values of Qe.
Nevertheless, the transformation of nonlinear equations to their
linear forms changes the error structure and can lead to violation of error
− 1

This fittingness test measures the difference between the experi­
mental adsorption capacities Qe,exp and the and model-calculated
adsorption capacities Qe,cal. The chi-square values (Table 3) of the
pseudo-second order model generally display lower values than the chisquare values for the first-order model and clearly indicate that the
pseudo-second order model fits better with the experimental data of the
metal ion (Cu2+, Zn2+) and dye (MB, RhB) adsorption with low χ2 values
6


C.R. Ehgartner et al.

Microporous and Mesoporous Materials 312 (2021) 110759

following [55].
Linearized Freundlich model:

Table 3
Pseudo-first and pseudo-second order linear and nonlinear kinetic parameters
for the adsorption of RhB, MB, Zn2+, and Cu2+ on MT80-T20 at room temper­
ature and a pH value of ~8.
Linear Pseudo-First Order Model

1
lnQe = lnKF + lnCe
n

Linear Pseudo-Second Order
Model
2

Linearized Langmuir model:
2

Adsorbate

K1
[minĂ1]

Qe
[mg.
gĂ1]

R

K2
[g.mgĂ1.
minĂ1]

Qe
[mg.
gĂ1]

R

RhB
MB
Zn2ỵ
Cu2ỵ

2.00E-04
3.00E-04
5.00E-04
2.00E-04

50.7
69.5
33.3
18.3

0.999
0.957
0.972

0.996

4.50E-05
2.20E-05
1.30E-04
1.50E-04

63.3
84
75.8
50.3

0.991
0.989
0.999
0.999

Nonlinear Pseudo-First Order Model

(8)

Ce
1
Ce
=
+
Qe Qm KL Qm

Where KF is the Freundlich isothermal constants and KL is the Langmuir
constant, respectively. 1/n is an indicator, if adsorption is favorable. Qm

(mg.g− 1) is the maximum adsorption capacity of the adsorbent. Table 4
and Fig. 4a–d clearly illustrate that the experimental data of all
adsorption isotherms fit better with the Langmuir model rather than the
Freundlich model. Correlation coefficients of R2 higher than 0.980 were
achieved for all metals and dye adsorption processes when fitted with
the Langmuir model. The correlation coefficients when fitting with the
Freundlich model were much lower (0.935–0.983). Moreover, the
maximum adsorption capacity calculated by the Langmuir equation
(Table 4) was close to the experimental results.

Nonlinear Pseudo-Second Order
Model

Adsorbate

K1
[minĂ1]

Qe
[mg.
gĂ1]

2

K2
[g.mgĂ1.
minĂ1]

Qe
[mg.

gĂ1]

2

RhB
MB
Zn2ỵ
Cu2ỵ

3.70E-03
1.80E-03
1.39E-02
3.50E-03

52.1
67.6
67.3
44.5

1.820
1.043
1.003
0.572

7.61E-05
2.51E-05
2.56E-04
1.40E-03

57.4

79.2
75.59
45.4

1.345
0.979
0.967
0.425

(9)

3.4. Selective adsorption and regeneration studies
between 0.425 and 1.345. The calculated adsorption capacity Qe
(pseudo-second order model, Table 3) additionally showed a better
agreement with the experimental values of Qe in comparison to the
pseudo-first order model.
For a better understanding of the interaction between the adsorbate
and absorbent, the respective adsorption isotherms were investigated at
room temperature. In Fig. 4 a-d the equilibrium adsorption capacity Qe
(mg.g− 1) is plotted against the equilibrium concentration Ce (mg.L− 1) of
RhB (Fig. 4a), MB (Fig. 4b), Zn2+ (Fig. 4c), and Cu2+ (Fig. 4d). With an
increasing value of Ce the adsorption capacity gradually increased for all
metal ion and dye solutions. There is an increased driving force from the
concentration gradient, which speeds up the diffusion of the metal ions
and dye molecules towards the aerogel [13]. Two most common line­
arized model equations for adsorption isotherms, namely Freundlich
and Langmuir were used to fit the experimental data as shown in the

The ability of MT80-T20 to selectively adsorb RhB or MB of a
cationic/anionic dye mixture was tested by two selective adsorption

Table 4
Langmuir and Freundlich adsorption isotherm parameters for the adsorption of
RhB, MB, Zn2+, and Cu2+ on MT80-T20 at room temperature and a pH value of
~8.
Langmuir

Freundlich

Adsorbate

KL
[L.
mgĂ1]

Qm
[mg.
gĂ1]

R2

KF
[mg1-n.Ln.
gĂ1]

1/n

R2

RhB
MB

Zn2ỵ
Cu2ỵ

0.015
0.141
0.011
0.185

154
106
111
78

0.981
0.993
0.989
0.980

4.3
1049.5
7.8
800.8

0.872
0.386
0.692
0.355

0.944
0.983

0.968
0.935

Fig. 4. Adsorption isotherms of (a) RhB, (b) MB, (c) Zn2+ and (d) Cu2+ on MT80-T20, including Langmuir and Freundlich model fit.
7


C.R. Ehgartner et al.

Microporous and Mesoporous Materials 312 (2021) 110759

experiments (Fig. 5). Fig. 5a and b display that the color of a RhB/TyG
(orange) or MB/TyG (green) solution changes after the adsorption
process and resembles the yellow color of the anionic TyG solution. The
molecular structures of the cationic/anionic dyes are displayed in
Table S3 (ESI). The UV–vis measurements underline this observation,
showing that the peak of RhB (λ = 554 nm, Fig. 5c) and MB (λ = 665 nm,
Fig. 5d) almost disappear after the adsorption process. The removal rate
of MB reaches 97% and RhB 96% after processing with MT80-T20 for 3
d.
Regeneration and recyclability of the adsorbent material were tested
by washing the MTMS monolith (MT80-T20) with 1 M HCl after the dye
adsorption treatment with MB. The low pH value of the HCl washing
step protonates the carboxyl groups of the samples, which releases the
MB dye out of the monolithic framework. The adsorption efficiency was
around 92% for three consecutive circles of adsorption and desorption of
MB and the removal efficiency remained at 88% and 87% for the 4th and
5th cycle (Figs. S8 and SI).

increasing particle sizes and the formation of macropores. Nevertheless,

the obtained carboxy-modified aerogels still possess low bulk densities
and high porosities for diverse catalytic and adsorption applications in
different media.
The pure MTMS sample mainly bear methyl (-CH3) groups on the
surface which render the monolith hydrophobic and prevent phys­
isorption of water on the surface. On the one hand, the methyl modified
framework gives the aerogel excellent mechanical abilities, making the
gels easier to handle, to dry ambiently, and to undergo repeated
adsorption processes in aqueous media. On the other hand, the hydro­
phobic character makes it harder for intended adsorption purposes.
Functionalization with carboxyl groups renders the hydrophobic MTMS
framework more hydrophilic. This has a direct influence on the
adsorption behavior of the functional materials. The still very strong
hydrophobic character of MT90-T10 prevents the adsorption of RhB
from aqueous solution, even though the aerogel has a relatively high
specific surface area. With a higher degree of modification, the interplay
between an increasing water wettability and the presence of an
increasing number of coordination sites enhances the adsorption per­
formance of the aerogels. Starting from the MT90-T10 sample, the
adsorption capacity increases up to a molar amount of TMPA of 20%
(MT80-T20) and then decreases for MT70-T30. Although MT70-T30
possess a higher surface loading with carboxyl groups, the capacity
difference is related to the lower specific surface area of MT70-T30 (36
m2.g− 1) compared to MT80-T20 (145 m2.g− 1).
The adsorption of metal ions occurs fast compared to the adsorption
of organic molecules. This can be attributed to the different sizes and
steric demands. The metal ions are relatively small in comparison to the
large MB and RhB molecules and are more easily transported to interior
adsorbent sites. The adsorption of RhB, MB, Zn2+, and Cu2+ on MT80T20 follows a pseudo-second order adsorption reaction in good agree­
ment of the calculated and the experimental values of the adsorption

capacity Qe. This is an indication that the adsorption on the aerogel
sample is controlled by chemical adsorption with a direct sharing and
exchange of electrons between adsorbents and adsorbate [56]. The
adsorption rate is therefore dominated by the availability of adsorption
sites and not by the concentration of the adsorbate [57,58]. Neverthe­
less, the linear adsorption kinetics of RhB corresponds slightly better to
the pseudo-first order model with a correlation coefficient of 0.999,
suggesting that in this case the adsorption depends on the adsorbate as
well as the sorbent and on chemisorption and physisorption processes
[52]. This behavior cannot be confirmed when applying the nonlinear
adsorption kinetic model, where the kinetic adsorption of RhB

3.4. Comparison to ambiently dried samples
To broaden the field of application of the carboxylic acidfunctionalized gels and to reduce the cost of the preparation, MT80T20 was also prepared via ambient pressure drying. Fig. S9 (SI) shows a
picture of the xerogel (MT80-T20x), which is translucent in appearance.
The bulk density (0.81 g.cm− 3) was higher and the porosity (34%) was
lower than the corresponding aerogel (0.46 g.cm− 3, 65%). The specific
surface area also decreased from 145 m2.g− 1 (aerogel) to 25 m2.g− 1
(xerogel). This is due to a higher degree of irreversible shrinkage due to
higher capillary forces experience during the drying step. Nevertheless,
the maximum adsorption capacity Qm (calculated by applying the
Langmuir model, Figs. S10 and SI) of the xerogel is still high with a value
of 85 mg.g− 1 for RhB (aerogel 154 mg g− 1).
4. Discussion
Carboxylic acid-modified MTMS polysilsesquioxane aerogels are
successfully prepared by a co-condensation approach of MTMS and
TMPA. The employment of the surfactant CTAB is essential to synthesize
stable monolithic aerogels since there is a great polarity difference be­
tween the hydrophobic MTMS and the hydrophilic TMPA. Additionally,
different hydrolysis and condensation rates of the silanes and an

increasing content of large functional moieties lead to a loss of homo­
geneity in the polysilsesquioxanes framework. This directly leads to

Fig. 5. Images of the selective adsorption of RhB from RhB/TyG (a) and MB from MB/TyG (b) mixed solutions using MT80-T20 and their corresponding UV–vis
spectra of the solutions before and after adsorption (c, d).
8


C.R. Ehgartner et al.

Microporous and Mesoporous Materials 312 (2021) 110759

corresponds better to the pseudo-second order model. This is a clear
indication that the nonlinear kinetic model is more accurate in
describing the kinetics of adsorption of the analyzed sample MT80-T20.
Nevertheless, the linear kinetic models are slightly better for predicting
the adsorption at equilibrium.
Adsorption isotherms of MT80-T20 against RhB, MB, Zn2+, and Cu2+
follow the Langmuir model rather than the Freundlich model. This
suggest a monolayer adsorption of metal ions and organic molecules on
the surface of the polysilsesquioxane [59]. Selective adsorption experi­
ments indicate that the functionalized polysilsesquioxane (MT80-T20)
displays excellent adsorption selectivity toward positively charged MB
and RhB ions from a mixed cationic/anionic solution. Additionally, the
carboxylic acid-modified samples are simple to regenerate and can be
repeatedly used for further adsorption experiments without significant
decrease in the removal efficiency and their form stability. The hydro­
phobicity and the mechanically stable network of the carboxy-modified
MTMS samples prevent the degradation during the repeated adsorption
processes in aqueous media. The loss in the adsorption capacities is

negligible and may be caused by MB being trapped inside some pores.
Comparison of the maximum adsorption capacity Qm of the MT80T20 aerogel towards RhB, MB, Zn2+, and Cu2+ with results reported in
literature show that the results of this study exceed the adsorption ca­
pacities of conventional adsorbents (like activated carbon and standard
silica aerogels) and are comparable to or succeed other functionalized
silica aerogels and other adsorption aerogels/materials (Table 5). The
maximum adsorption capacity Qm of the ambient dried MT80-T20x
xerogel towards RhB is also comparable to good adsorption materials
(Table 5). This indicates that the functionalization with 5-(trimethox­
ysilyl)pentanoic acid has a huge impact on the adsorption capabilities of
the xerogels and aerogels, whereas the surface area is not as relevant in
comparison.
The highest adsorption capacity in this study was obtained for RhB
with Qm being 154 mg.g-1. The positively charged RhB ion (which also
has carboxyl groups) has a higher positive charge density than the
respective MB molecule and metal ions. In addition, the adsorption ef­
fect of RhB towards the polysilsesquioxane is enhanced by hydrogen
bonds due to the presence of electric donors and acceptors (from its
carboxyl groups). The positively charged MB molecule and the metal
ions do not have electronic donors or receptors and the adsorption is
mainly dominated by weaker Van der Waals electrostatic interactions
[53]. The general good values for the adsorption capacity indicates that
the carboxylic acid-modified polysilsesquioxane aerogels are efficient

adsorbents for organic dyes and can be used for heavy metal complex­
ation reactions. The adsorption performance depends on the adsorbate
molecule/ion size, the specific surface area of the adsorbents as well as
on electrostatic/ionic interactions between the carboxylic surface
groups and the metal ions and dyes [60].
5. Conclusion

Tunable carboxylic acid-modified aerogels have been synthesized
from a simple co-condensation approach of MTMS with 5-(trimethox­
ysilyl)pentanoic acid via an acid-base catalyzed sol-gel process
employing CTAB as a phase separation suppressing surfactant. The one
pot approach at RT is time and energy efficient in comparison to tedious
grafting procedures. The mesoporous functionalized silica aerogels
possess low densities, high porosities and large specific surface areas
that are dependent on the degree of functionalization. Moreover, a high
density of active and available carboxyl groups is present which makes
the monoliths suitable for adsorption of organic molecules and
complexation reactions with metal ions. Carboxylic acid-modified
MTMS aerogel samples showed maximum adsorption capacities for
RhB, MB, Zn2+, and Cu2+ of 154 mg.g− 1, 106 mg.g− 1, 111 mg g− 1, and
78 mg.g− 1, respectively. The adsorption capacity is comparable or ex­
ceeds commercial adsorbents (like activated carbon and standard silica
aerogels) with the main advantage of being selective. The aerogels
selectively adsorb positive charged molecules and metal cations, which
makes them ideal candidates for selective adsorption processes in
different media. Furthermore, the materials have a hydrophobic char­
acter and are stable for adsorption processes in water and can be easily
regenerated by a simple acid washing process where both the adsorbate
and the absorbent can be recovered. The hydrophobic abilities make the
aerogels also ideal adsorbents for oil spillage and organic solvents.
Additionally, the aerogels can also be dried under ambient conditions,
with good and comparable adsorption capacities, which greatly reduces
the cost of production and broadens their field of application.
CRediT authorship contribution statement
C.R. Ehgartner: Conceptualization, Methodology, Validation,
Formal analysis, Investigation, Writing - original draft. V. Werner:
Validation, Investigation. S. Selz: Investigation. N. Hüsing: Resources,

Writing - review & editing. A. Feinle: Conceptualization, Writing - re­
view & editing, Supervision.

Table 5
Comparison of maximum adsorption capacity Qm of RhB, MB, Zn2+ and Cu2+ with various adsorbents reported in literature.
Adsorbent

RhB Qm
[mg.gĂ1]

MB Qm
[mg.gĂ1]

Cu2ỵ Qm
[mg.gĂ1]

Zn2ỵ Qm
[mg.gĂ1]

SBET[a]
[m2.gĂ1]

Reference

Carboxy-functionalized MTMS aerogel MT80-T20
Carboxy-functionalized MTMS xerogel MT80-T20x
Hydrophobic surface modified silica aerogel
Amine-functionalized silica aerogel
Amine-functionalized silica aerogel
Mercapto-functionalized silica aerogel

Mercapto-functionalized silica aerogel
EDTA-functionalized silica
Magnetic amine-functionalized mesoporous Silica
Phenyl-functionalized silica aerogel
Silica–titania composite aerogel
Carboxy-functionalized mesoporous silica
Carboxy-functionalized indole-based aerogel
Poly(methacrylic acid-co-maleic acid) grafted nano fibrillated cellulose aerogel
Microcrystalline cellulose aerogel modified with PDA
Sodium alginate aerogel
Activated carbon
Carbon aerogel
Graphite aerogel

154
85
134
–
–
–
–
–
–
–
26
–
–
–
–
–

61
~100
281

106
–
65
–
–
–
–
–
–
49
–
113
–
–
153
–
14
~38
–

78
–
–
117
48
51

83
79
93
–
–
–
383
–
–
87.83
6.8
–
–

111
–
–
100
–
–
–
74
82
–
–
–
217
136
–
–

3.9
–
–

145
25
626
329
240
518
388
3
880
–
914
757
143
65
37
–
550–1160
610
–

This work
This work
[53]
[61]
[62]
[22]

[63]
[64]
[65]
[52]
[66]
[29]
[67]
[68]
[55]
[69]
[70–72]
[73]
[74]

a

Specific surface area determined using the BET model.

9


C.R. Ehgartner et al.

Microporous and Mesoporous Materials 312 (2021) 110759

Declaration of competing interest

[17] H. Maleki, L. Dur˜
aes, A. Portugal, An overview on silica aerogels synthesis and
different mechanical reinforcing strategies, J. Non-Cryst. Solids 385 (2014) 55–74,

/>[18] C. Zhang, J. Su, H. Zhu, J. Xiong, X. Liu, D. Li, Y. Chen, Y. Li, The removal of heavy
metal ions from aqueous solutions by amine functionalized cellulose pretreated
with microwave-H2O2, RSC Adv. 7 (2017) 34182–34191, />10.1039/c7ra03056h.
[19] Y. Yang, Q. Zhang, Z. Zhang, S. Zhang, Functional microporous polyimides based
on sulfonated binaphthalene dianhydride for uptake and separation of carbon
dioxide and vapors, J. Mater. Chem. A. 1 (2013) 10368–10374, />10.1039/c3ta11621b.
[20] J.Y. Lee, C.H. Chen, S. Cheng, H.Y. Li, Adsorption of Pb(II) and Cu(II) metal ions on
functionalized large-pore mesoporous silica, Int. J. Environ. Sci. Technol. 13
(2016) 65–76, />[21] P.O. Boamah, Y. Huang, M. Hua, Q. Zhang, J. Wu, J. Onumah, L.K. Sam-Amoah,
Sorption of heavy metal ions onto carboxylate chitosan derivatives-A mini-review,
Ecotoxicol. Environ. Saf. 116 (2015) 113–120, />ecoenv.2015.01.012.
ˇ
ˇ Knez, Silica aerogels modified with
[22] S. Standeker,
A. Veronovski, Z. Novak, Z.
mercapto functional groups used for Cu(II) and Hg(II) removal from aqueous
solutions, Desalination 269 (2011) 223–230, />desal.2010.10.064.
[23] Z. Ali, A. Khan, R. Ahmad, The use of functionalized aerogels as a low level
chromium scavenger, Microporous Mesoporous Mater. 203 (2015) 8–16, https://
doi.org/10.1016/j.micromeso.2014.10.004.
[24] G. Hayase, K. Kanamori, M. Fukuchi, H. Kaji, K. Nakanishi, Facile synthesis of
Marshmallow-like macroporous gels usable under harsh conditions for the
separation of oil and water, Angew. Chem. 125 (2013) 2040–2043, https://doi.
org/10.1002/ange.201207969.
[25] G. Hayase, K. Kanamori, G. Hasegawa, A. Maeno, H. Kaji, K. Nakanishi,
A superamphiphobic macroporous silicone monolith with Marshmallow-like
flexibility, Angew. Chem. 125 (2013) 10988–10991, />ange.201304169.
[26] T. Bala, B.L.V. Prasad, M. Sastry, M.U. Kahaly, U.V. Waghmare, Interaction of
different metal ions with carboxylic acid group: a quantitative study, J. Phys.
Chem. 111 (2007) 6183–6190, />[27] J. Kecht, A. Schlossbauer, T. Bein, Selective functionalization of the outer and inner

surfaces in mesoporous silica nanoparticles, Chem. Mater. (2008) 7207–7214,
/>[28] X. Fu, X. Chen, J. Wang, J. Liu, Fabrication of carboxylic functionalized
superparamagnetic mesoporous silica microspheres and their application for
removal basic dye pollutants from water, Microporous Mesoporous Mater. 139
(2011) 8–15, />[29] K.Y. Ho, G. McKay, K.L. Yeung, Selective adsorbents from ordered mesoporous
silica, Langmuir 19 (2003) 3019–3024, />80581-0.
[30] L. Han, O. Terasaki, S. Che, Carboxylic group functionalized ordered mesoporous
silicas, J. Mater. Chem. 21 (2011) 11033–11039, />c1jm10561b.
[31] L. Han, Y. Sakamoto, O. Terasaki, Y. Li, S. Che, Synthesis of carboxylic group
functionalized mesoporous silicas (CFMSs) with various structures, J. Mater. Chem.
17 (2007) 1216–1221, />[32] C.T. Tsai, Y.C. Pan, C.C. Ting, S. Vetrivel, A.S.T. Chiang, G.T.K. Fey, H.M. Kao,
A simple one-pot route to mesoporous silicas SBA-15 functionalized with
exceptionally high loadings of pendant carboxylic acid groups, Chem. Commun.
(2009) 5018–5020, />[33] M.A. Markowitz, J. Klaehn, R.A. Hendel, S.B. Qadriq, S.L. Golledge, D.G. Castner,
B.P. Gaber, Direct synthesis of metal-chelating mesoporous silica: effects of added
organosilanes on silicate formation and adsorption properties, J. Phys. Chem. B
104 (2000) 10820–10826, />[34] N.A. Brunelli, K. Venkatasubbaiah, C.W. Jones, Cooperative catalysis with acidbase bifunctional mesoporous silica: impact of grafting and Co-condensation
synthesis methods on material structure and catalytic properties, Chem. Mater. 24
(2012) 2433–2442, />[35] D.R. Radu, C.Y. Lai, J. Huang, X. Shu, V.S.Y. Lin, Fine-tuning the degree of organic
functionalization of mesoporous silica nanosphere materials via an interfacially
designed co-condensation method, Chem. Commun. (2005) 1264–1266, https://
doi.org/10.1039/b412618a.
[36] A. Feinle, S. Flaig, M. Puchberger, U. Schubert, N. Hüsing, Stable carboxylic acid
derivatized alkoxy silanes, Chem. Commun. 51 (2015) 2339–2341, https://doi.
org/10.1039/c4cc08025d.
[37] A. Feinle, F. Lavoie-Cardinal, J. Akbarzadeh, H. Peterlik, M. Adlung, C. Wickleder,
N. Ḧsing, Novel sol-gel precursors for thin mesoporous Eu 3+-doped silica coatings
as efficient luminescent materials, Chem. Mater. 24 (2012) 3674–3683, https://
doi.org/10.1021/cm300996j.
[38] A. Feinle, F. Leichtfried, S. Straßer, N. Hüsing, Carboxylic acid-functionalized

porous silica particles by a co-condensation approach, J. Sol. Gel Sci. Technol. 81
(2017) 138–146, />[39] H. Maleki, L. Whitmore, N. Hüsing, Novel multifunctional
polymethylsilsesquioxane-silk fibroin aerogel hybrids for environmental and
thermal insulation applications, J. Mater. Chem. A. 6 (2018) 12598–12612,
/>[40] H. Maleki, S. Montes, N. Hayati-Roodbari, F. Putz, N. Huesing, Compressible,
Thermally insulating, and fire retardant aerogels through self-assembling silk
fibroin biopolymers inside a silica structure - an approach towards 3D printing of

The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Acknowledgements
The authors thank M. Suljic for nitrogen adsorption/desorption
measurements and R. Torres for TEM measurements performed at the
University of Salzburg. N. H. gratefully acknowledges financial support
ă
from Interreg Osterreich-Bayern
20142010 Project AB29 Synthese,
ătze fỹr den
Charakterisierung und technologische Fertigungsansa
Leichtbau n2m (nano-to-macro).
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.micromeso.2020.110759.
References
[1] I. Ali, New generation adsorbents for water treatment, Chem. Rev. 112 (2012)
5073–5091, />[2] V. Katheresan, J. Kansedo, S.Y. Lau, Efficiency of various recent wastewater dye
removal methods: a review, J. Environ. Chem. Eng. 6 (2018) 4676–4697, https://
doi.org/10.1016/j.jece.2018.06.060.
[3] A.E. Burakov, E.V. Galunin, I.V. Burakova, A.E. Kucherova, S. Agarwal, A.

G. Tkachev, V.K. Gupta, Adsorption of heavy metals on conventional and
nanostructured materials for wastewater treatment purposes: a review, Ecotoxicol.
Environ. Saf. 148 (2018) 702–712, />ecoenv.2017.11.034.
[4] N. Shao, S. Tang, Z. Liu, L. Li, F. Yan, F. Liu, S. Li, Z. Zhang, Hierarchically
structured calcium silicate hydrate-based nanocomposites derived from steel slag
for highly efficient heavy metal removal from wastewater, ACS Sustain. Chem. Eng.
6 (2018) 14926–14935, />[5] M. Hu, B. Mi, Enabling graphene oxide nanosheets as water separation membranes,
Environ. Sci. Technol. 47 (2013) 3715–3723, />[6] Y. Han, Z. Xu, C. Gao, Ultrathin graphene nanofiltration membrane for water
purification, Adv. Funct. Mater. 23 (2013) 3693–3700, />adfm.201202601.
[7] D. Chandra, S.K. Das, A. Bhaumik, A fluorophore grafted 2D-hexagonal
mesoporous organosilica: excellent ion-exchanger for the removal of heavy metal
ions from wastewater, Microporous Mesoporous Mater. 128 (2010) 34–40, https://
doi.org/10.1016/j.micromeso.2009.07.024.
[8] T. Huang, Y.W. Shao, Q. Zhang, Y.F. Deng, Z.X. Liang, F.Z. Guo, P.C. Li, Y. Wang,
Chitosan-cross-linked graphene oxide/carboxymethyl cellulose aerogel globules
with high structure stability in liquid and extremely high adsorption ability, ACS
Sustain. Chem. Eng. 7 (2019) 87758788, />acssuschemeng.9b00691.
[9] S.M. Alatalo, F. Pileidis, E. Mă
akilă
a, M. Sevilla, E. Repo, J. Salonen, M. Sillanpă
aă
a, M.
M. Titirici, Versatile cellulose-based carbon aerogel for the removal of both
cationic and anionic metal contaminants from water, ACS Appl. Mater. Interfaces 7
(2015) 25875–25883, />[10] J. Ali, H. Wang, J. Ifthikar, A. Khan, T. Wang, K. Zhan, A. Shahzad, Z. Chen,
Z. Chen, Efficient, stable and selective adsorption of heavy metals by thiofunctionalized layered double hydroxide in diverse types of water, Chem. Eng. J.
332 (2018) 387–397, />[11] P.L. Yap, S. Kabiri, D.N.H. Tran, D. Losic, Selective and highly efficient adsorption
of mercury, ACS Appl. Mater. Interfaces 11 (2019) 6350–6362, />10.1021/acsami.8b17131.
[12] J. Fu, Q. Xin, X. Wu, Z. Chen, Y. Yan, S. Liu, M. Wang, Q. Xu, Selective adsorption
and separation of organic dyes from aqueous solution on polydopamine

microspheres, J. Colloid Interface Sci. 461 (2016) 292–304, />10.1016/j.jcis.2015.09.017.
[13] Z. Dong, D. Wang, X. Liu, X. Pei, L. Chen, J. Jin, Bio-inspired surfacefunctionalization of graphene oxide for the adsorption of organic dyes and heavy
metal ions with a superhigh capacity, J. Mater. Chem. A. 2 (2014) 5034–5040,
/>[14] K. Li, M. Zhou, L. Liang, L. Jiang, W. Wang, Ultrahigh-surface-area activated
carbon aerogels derived from glucose for high-performance organic pollutants
adsorption, J. Colloid Interface Sci. 546 (2019) 333–343, />j.jcis.2019.03.076.
[15] S. Kabiri, D.N.H. Tran, S. Azari, D. Losic, Graphene-diatom silica aerogels for
efficient removal of mercury ions from water, ACS Appl. Mater. Interfaces 7 (2015)
11815–11823, />[16] A. Walcarius, L. Mercier, Mesoporous organosilica adsorbents: nanoengineered
materials for removal of organic and inorganic pollutants, J. Mater. Chem. 20
(2010) 4478–4511, />
10


C.R. Ehgartner et al.

[41]
[42]
[43]
[44]

[45]
[46]

[47]

[48]
[49]
[50]
[51]

[52]
[53]

[54]

[55]

[56]
[57]

Microporous and Mesoporous Materials 312 (2021) 110759
[58] E.G. Deze, S.K. Papageorgiou, E.P. Favvas, F.K. Katsaros, Porous alginate aerogel
beads for effective and rapid heavy metal sorption from aqueous solutions: effect of
porosity in Cu2+ and Cd2+ ion sorption, Chem. Eng. J. 209 (2012) 537–546,
/>[59] C.H. Tsai, W.C. Chang, D. Saikia, C.E. Wu, H.M. Kao, Functionalization of cubic
mesoporous silica SBA-16 with carboxylic acid via one-pot synthesis route for
effective removal of cationic dyes, J. Hazard Mater. 309 (2016) 236–248, https://
doi.org/10.1016/j.jhazmat.2015.08.051.
[60] W. Wei, H. Hu, X. Ji, Z. Yan, W. Sun, J. Xie, Selective adsorption of organic dyes by
porous hydrophilic silica aerogels from aqueous system, Water Sci. Technol. 78
(2018) 402–414, />[61] J.P. Vareda, L. Dur˜
aes, Efficient adsorption of multiple heavy metals with tailored
silica aerogel-like materials, Environ. Technol. 40 (2019) 529–541, https://doi.
org/10.1080/09593330.2017.1397766.
[62] H. Faghihian, H. Nourmoradi, M. Shokouhi, Removal of copper (II) and nickel (II)
from aqueous media using silica aerogel modified with amino propyl
triethoxysilane as an adsorbent: equilibrium, kinetic, and isotherms study,
Desalination Water Treat. 52 (2014) 305–313, />19443994.2013.785367.
[63] H.R. Pouretedal, M. Kazemi, Characterization of modified silica aerogel using
sodium silicate precursor and its application as adsorbent of Cu2+, Cd2+, and Pb2

+ ions, Int. J. Ind. Chem. 3 (2012) 1, />[64] R. Kumar, M.A. Barakat, Y.A. Daza, H.L. Woodcock, J.N. Kuhn, EDTA
functionalized silica for removal of Cu(II), Zn(II) and Ni(II) from aqueous solution,
J. Colloid Interface Sci. 408 (2013) 200–205, />jcis.2013.07.019.
[65] A. Mehdinia, S. Shegefti, F. Shemirani, Removal of lead(II), copper(II) and zinc(II)
ions from aqueous solutions using magnetic amine-functionalized mesoporous
silica nanocomposites, J. Braz. Chem. Soc. 26 (2015) 2249–2257, />10.5935/0103-5053.20150211.
[66] Y. Yu, M. Zhu, W. Liang, S. Rhodes, J. Fang, Synthesis of silica-titania composite
aerogel beads for the removal of Rhodamine B in water, RSC Adv. 5 (2015)
72437–72443, />[67] L. Yang, P. Yang, Y. Ma, G. Chang, A novel carboxylic-functional indole-based
aerogel for highly effective removal of heavy metals from aqueous solution via
synergistic effects of face-point and point-point interactions, RSC Adv. 9 (2019)
24875–24879, />[68] W. Maatar, S. Boufi, Poly(methacylic acid-co-maleic acid) grafted nanofibrillated
cellulose as a reusable novel heavy metal ions adsorbent, Carbohydr. Polym. 126
(2015) 199–207, />[69] Y. Huang, Z. Wang, Preparation of composite aerogels based on sodium alginate,
and its application in removal of Pb2+and Cu2+from water, Int. J. Biol. Macromol.
107 (2018) 741–747, />[70] S. Jiang, L. Huang, T.A.H. Nguyen, Y.S. Ok, V. Rudolph, H. Yang, D. Zhang, Copper
and zinc adsorption by softwood and hardwood biochars under elevated sulphateinduced salinity and acidic pH conditions, Chemosphere 142 (2016) 64–71,
/>[71] S. Yenisoy-Karakas¸, A. Aygün, M. Günes¸, E. Tahtasakal, Physical and chemical
characteristics of polymer-based spherical activated carbon and its ability to
adsorb organics, Carbon N. Y. 42 (2004) 477–484, />carbon.2003.11.019.
[72] M. Hema, S. Arivoli, Rhodamine B adsorption by activated carbon: kinetic and
equilibrium studies, Indian J. Chem. Technol. 16 (2009) 38–45.
[73] W. Yang, D. Wu, R. Fu, Effect of surface chemistry on the adsorption of basic dyes
on carbon aerogels, Colloids Surf. A Physicochem. Eng. Asp. 312 (2008) 118–124,
/>[74] C. Liu, H. Liu, A. Xu, K. Tang, Y. Huang, C. Lu, In situ reduced and assembled threedimensional graphene aerogel for efficient dye removal, J. Alloys Compd. 714
(2017) 522–529, />
aerogels, ACS Appl. Mater. Interfaces 10 (2018) 22718–22730, />10.1021/acsami.8b05856.
C.R. Ehgartner, S. Grandl, A. Feinle, N. Hüsing, Flexible organofunctional aerogels,
Dalton Trans. 46 (2017) 8809–8817, />K. Kanamori, M. Aizawa, K. Nakanishi, T. Hanada, New transparent
methylsilsesquioxane aerogels and xerogels with improved mechanical properties,

Adv. Mater. 19 (2007) 1589–1593, />K. Kanamori, K. Nakanishi, Controlled pore formation in organotrialkoxysilanederived hybrids: from aerogels to hierarchically porous monoliths, Chem. Soc. Rev.
40 (2011) 754–770, />P.R. Aravind, P. Niemeyer, L. Ratke, Novel flexible aerogels derived from
methyltrimethoxysilane/3-(2,3- epoxypropoxy)propyltrimethoxysilane coprecursor, Microporous Mesoporous Mater. 181 (2013) 111–115, />10.1016/j.micromeso.2013.07.025.
G. Hayase, K. Kanamori, K. Nakanishi, New flexible aerogels and xerogels derived
from methyltrimethoxysilane/dimethyldimethoxysilane co-precursors, J. Mater.
Chem. 21 (2011) 17077–17079, />M. Thommes, K. Kaneko, A.V. Neimark, J.P. Olivier, F. Rodriguez-Reinoso,
J. Rouquerol, K.S.W. Sing, Physisorption of gases, with special reference to the
evaluation of surface area and pore size distribution (IUPAC Technical Report),
Pure Appl. Chem. 87 (2015) 1051–1069, />L.B. Capeletti, I.M. Baibich, I.S. Butler, J.H.Z. Dos Santos, Infrared and Raman
spectroscopic characterization of some organic substituted hybrid silicas,
Spectrochim. Acta Part A Mol. Biomol. Spectrosc. 133 (2014) 619–625, https://
doi.org/10.1016/j.saa.2014.05.072.
S. Fiorilli, B. Onida, B. Bonelli, E. Garrone, In situ infrared study of SBA-15
functionalized with carboxylic groups incorporated by a Co-condensation route,
J. Phys. Chem. B 109 (2005) 16725–16729, />N. Hüsing, U. Schubert, Organofunctional silica aerogels, J. Sol. Gel Sci. Technol. 8
(1997) 807–812, />K.B. Krauskopf, Disolution and precipitation of silica at low temperatures,
Geochem. Cosmochim. Acta 10 (1956) 1–26.
F.K. Crundwell, On the mechanism of the dissolution of quartz and silica in
aqueous solutions, ACS Omega 2 (2017) 1116–1127, />acsomega.7b00019.
N. Saad, M. Al-Mawla, E. Moubarak, M. Al-Ghoul, H. El-Rassy, Surfacefunctionalized silica aerogels and alcogels for methylene blue adsorption, RSC Adv.
5 (2015) 6111–6122, />H. Han, W. Wei, Z. Jiang, J. Lu, J. Zhu, J. Xie, Removal of cationic dyes from
aqueous solution by adsorption onto hydrophobic/hydrophilic silica aerogel,
Colloids Surf. A Physicochem. Eng. Asp. 509 (2016) 539–549, />10.1016/j.colsurfa.2016.09.056.
J. L´
opez-Luna, L.E. Ramírez-Montes, S. Martinez-Vargas, A.I. Martínez, O.
F. Mijangos-Ricardez, M.d.C.A. Gonz´
alez-Ch´
avez, R. Carrillo-Gonz´
alez, F.A. SolísDomínguez, M.d.C. Cuevas-Díaz, V. V´
azquez-Hip´

olito, Linear and nonlinear kinetic
and isotherm adsorption models for arsenic removal by manganese ferrite
nanoparticles, Appl. Sci. 1 (2019) 1–19, />X. Wei, T. Huang, J. Nie, J. hui Yang, X. dong Qi, Z. wan Zhou, Y. Wang, Bioinspired functionalization of microcrystalline cellulose aerogel with high
adsorption performance toward dyes, Carbohydr. Polym. 198 (2018) 546–555,
/>X. Mi, G. Huang, W. Xie, W. Wang, Y. Liu, J. Gao, Preparation of graphene oxide
aerogel and its adsorption for Cu 2+ ions, Carbon N. Y. 50 (2012) 4856–4864,
/>Y. Liu, New insights into pseudo-second-order kinetic equation for adsorption,
Colloids Surf. A Physicochem. Eng. Asp. 320 (2008) 275–278, />10.1016/j.colsurfa.2008.01.032.

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